Dispensing cartridges of this nature serve in known manner for holding and processing pasty substances or those having medium to low viscosity, which upon the action on the delivery piston are driven out through the cartridge outlet. When working with such cartridges, one difficulty encountered is that the content tends to continue to flow after the action of the dispensing plunger on the delivery piston has ceased. This phenomenon is undesirable and irksome, since it prevents working neatly and does not permit precisely apportioned dispensing of small quantities. This continued flow is primarily due to the reservoir cylinder of the cartridge (most frequently built as a thin-walled disposable item of synthetic material), which, when dispensing, "breathes". That is, the cylinder is elastically expanded from the inside under the pressure of the cartridge content during piston advance and, upon cessation of the dispensing pressure, again assumes its starting shape. The delivery piston, however, due to the considerable sealing friction against the cylinder wall comes to a standstill. These phenomena are particularly marked and disturbing in so-called double cartridges for two-component substances like adhesive or sealing agents, etc., with flow mixers connected to the cartridge outlet. In such cartridges, due to the presence of the mixer, the outflow resistance offered the substances and, consequently, the pressure inside the cartridge required for dispensing is especially high. This, consequently, requires relatively high pressure of the piston sealing against the cylinder wall as well as correspondingly high plunger forces on the delivery pistons.
In French Pat. No. 1 263 356, a dispensing cartridge of the above-mentioned kind is disclosed, where, on the single-piece delivery piston, the piston head is arched toward the dispensing plunger and the sealing is formed by the cylindrical piston wall. During each plunger advance, the piston head is flattened by which an increased sealing effect is to be achieved due to the radial expansion of the piston wall; in addition, the piston head becomes arched again after completion of the plunger stroke, so that afterflow of the cartridge content is supposed to be avoided. Here, however, the volumetric change, which can be brought about by the deformation of the piston head alone, is very limited and, particularly when the cartridge is still full, is hardly sufficient to compensate for the "breathing" and to prevent afterflow completely. A further disadvantage is that a radial form change of the piston wall cannot be controlled through the mentioned effect of the piston head to the extent that, given the existing production tolerances, complete sealing of the piston is ensured during the advance as well as in the resting state. If the latter is to be achieved here, a high degree of piston friction inevitably occurs.
In another known dispensing cartridge of the mentioned kind (French Pat. No. 2, 097 755) for the single-piece delivery piston, the piston head is arched toward the cartridge content; at the outer edge of the piston head, a sealing lip is integrally attached, and the piston wall rests at a distance from the sealing lip slidingly movable against the cartridge wall. The purpose of this piston design is prevention of air enclosures when closing the filled cartridge. On dispensing the cartridge content during each plunger advance, the piston head is elastically deformed between the plunger disk and the content, i.e., it is partially flattened, in order to press the sealing lip during advance radially against the cartridge wall and subsequently, with the piston head unstressed and returned to original form to be lifted again; in the resting state, sealing is then effected solely through the above mentioned piston wall section. Preventing continued flow (afterflow) of the cartridge content in this design is neither intended nor possible, on the contrary, the afterflow is increased in that the piston head arches back against the cartridge content and, in so doing, decreases the available cartridge volume.
The task of the present invention consists in effectively preventing, through suitable design of the dispensing cartridge, in particular of the delivery piston, uncontrolled afterflow of the cartridge content after each partial dispensing and simultaneously keeping the piston friction at a minimum without sacrificing sealing in the stressed as well as in the unstressed state.